The present invention relates to a tone signal generator and more particularly, to a tone signal generator which is capable of producing frequency modulation (FM) multioperator tone signals.
A digital tone signal generator is a general electronic circuit which generates polyphonic digital output signals representing music and other types of sound. An output digital-to-analog converter ("DAC") converts the tone signal generator's digital output signals into analog signals. A speaker converts the analog signals into sound waves. Chowning, U.S. Pat. No. 4,018,121 is an example of a device which synthesizes sound using frequency modulation.
In generating the polyphonic digital output signals, it is known to use a plurality of operators (also known as operation units or operation channels) to generate a tone signal having a particular tone color. The number and type of operators as well as the manner in which these operators are combined dictate the particular tone color. For example, Samson, "A General-Purpose Digital Synthesizer," J. Aud. Eng. Soc., March 1980, pp. 106-113 describes a digital musical synthesizer which includes a plurality of building blocks of digital synthesis.
It is known to use feedback to adjust the tone color of a tone signal. For example, FIG. 1, labeled prior art, shows an operator which includes a feedback path for adjusting the tone color of a tone signal. In this system, an operator circuit, which includes a waveform generator, receives the sum of a modulation input signal (m(t)) and a tone feedback signal. The operator circuit provides a tone signal (e(t)) based upon the modulation input signal m(t) and the tone feedback signal. The tone signal is multiplied by an amplitude signal (A(t)) as well as by a feedback value (.beta.) to provide the tone feedback signal. An example of a tone generator which includes the use of feedback is the tone generator disclosed in Tomisawa, U.S. Pat. No. 4,249,447. However, while Tomisawa includes an averaging circuit with corresponding memory and shows an amplitude multiplier for performing the amplitude multiplication, this amplitude multiplier is not located in the feedback path.
It is also known to store progressive phase angle samples of a waveshape in a waveshape table and to use these samples to derive a carrier or modulating wave signal. For example, Hirano et al., U.S. Pat. No. 4,813,326, discloses synthesizing music tones where the modulating wave and carrier wave are derived from waveshape samples that are stored within respective wave tables.